1. Field of the Invention
The present invention relates to a spectroscopy apparatus and method, and preferably to a laser-ultrasound spectroscopy apparatus and method for detecting, measuring, or identifying one or more shear resonances in an object, and for calculating physical properties of the object, including, for example, thickness, anisotropy, and texture (i.e., the crystallographic orientation distribution), in accordance with the detected shear resonances.
2. Description of the Related Art
Ultrasonics generally refers to the principle of generating ultrasonic vibrations in an object, and then detecting the vibrations to determine the geometrical, microstructural, and physical properties of the object. This technique is advantageous because it is nondestructive.
Conventional ultrasound devices have been developed for detecting geometrical, microstructural, and physical properties of objects. One conventional ultrasound approach involves the use of transducers, including piezoelectric and electromagnetic acoustic transducers (EMATs). Examples include those discussed in U.S. Pat. No. 4,790,188 (Bussiere, et al.), U.S. Pat. Nos. 4,899,589 (Thompson, et al.) and 5,251,486 (Thompson, et al.), and Thompson, et al., "Angular Dependence of Ultrasonic Wave Propagation in a Stressed Orthorhombic Continuum: Theory and Application to the Measurement of Stress and Texture", J. Acoust. Soc. Am., Vol. 80, No. 3 (September 1996), pp. 921-931. In these conventional approaches, texture (i.e., the crystallographic orientation distribution) or other properties are evaluated by measuring angular variations of elastic wave velocities or plate mode velocities. However, a number of drawbacks exist. First, these conventional approaches require sending and receiving ultrasound in various directions in the plane of a plate, for example, at 0, 45, and 90 degrees to the rolling direction. Secondly, the piezoelectric approach requires immersion or water coupling, or contact with the object being tested, while the EMAT approach, although non-contact, requires the sensor to be very close to the sheet surface, e.g., 1 mm, and these requirements could cause problems if the object or metal sheet is hot, lifts off when moving, or presents irregularities such as welds between two pieces. Thirdly, conventional ultrasonic devices are often subject to precise orientation requirements, with angular tolerance being a few degrees or less, meaning that the inspection of curved surfaces requires a surface contour following device.
Another conventional approach is the electromagnetic acoustic resonance (EMAR) technique, as exemplified by (a) U.S. Pat. No. 5,467,655 (Hyoguchi, et al.); (b) Kawashima, "Nondestructive Characterization of Texture and Plastic Strain Ratio of Metal Sheets with Electromagnetic Acoustic Transducers", J. Acoust. Soc. Am., Vol. 87, No. 2 (February 1990), pp. 681-690; and (c) Kawashima, et al., "On-Line Measurement of Plastic Strain Ratio of Steel Sheet Using Resonance Mode EMAT", Journal of Nondestructive Evaluation, Vol. 12, No. 1 (1993), pp. 71-77. The EMAR technique generally involves exciting the EMAT by RF (radio frequency) narrow-band tonebursts while sequentially sweeping through a frequency range of interest. However, drawbacks exist in that this approach requires an EMAT, which suffers from the problems described above, and in that this approach requires multiple measurements to cover a wide frequency bandwidth.
Another ultrasound technique is laser-ultrasonics, wherein one laser is used to generate ultrasonic vibrations in an object, and another is used for detection. Either laser may be coupled through an optical fiber for ease of handling. Examples of this approach include those discussed in U.S. Pat. Nos. 4,659,224 (Monchalin) and 4,966,459 (Monchalin). This approach is advantageous because it does not require either the generation laser or the laser-interferometer detector to be close to the sheet. Furthermore, unlike an EMAT or piezoelectric transducer, the generation laser and laser-interferometer are not subject to precise orientation requirements, because their operation is substantially orientation insensitive. Conventional laser-ultrasound devices have been used, for example, to measure time of flight between two longitudinal echoes, and from that measurement to determine thickness (e.g., Jean-Pierre Monchalin, et al., "Wall Thickness Measurement of Tubes and Eccentricity Determination by Laser-Ultrasonics", 39th Mechanical Working & Steel Processing Conference, Iron & Steel Society, Indianapolis, Ind., Oct. 19-22, 1997, Iron & Steel Society, Warrendale, Pa.).
However, we have found that conventional ultrasound devices have not been able to determine thickness corrected for texture. Thickness corrected for texture (also referred to as independent of texture) is an ultrasonic measurement of thickness which takes into account the crystallographic orientation distribution of the object. Texture here refers to the crystallographic orientation distribution of polycrystalline aggregates. Because the ultrasonic velocity in a single crystal depends on the direction of propagation within the single crystal, the ultrasonic velocity in a polycrystalline aggregate depends on the average orientation of the crystallites with respect to the propagation direction of the ultrasound. Variations of the crystallographic orientation distribution (texture variations) commonly found in metals can cause variations of the ultrasonic velocity in excess of 1%, thus limiting thickness measurement accuracy using time of flight measurements to 1% at best.
Texture measurements are also useful measurements in determining a large number of physical properties, such as ductility and formability (the ability to plastically deform), and the anisotropy of physical properties such as tensile strength, which are of great importance to manufacturing industries (as discussed in Murayama, et al., "Development of an On-line Evaluation System of Formability in Cold-rolled Steel Sheets Using Electromagnetic Acoustic Transducers (EMATs)", NDT&E International, Vol. 29 (1996), pp. 141-146; and Bunge, "Texture Analysis in Materials Science", Butterworth & Co. (1982)).
We have found that to improve thickness measurement accuracy or to make some texture measurements with ultrasonics, it is necessary to measure shear acoustic wave velocities, and one way to measure these velocities is to search for resonances in an object. However, we have found that laser-ultrasound devices have not been able to detect and measure shear resonances in an object.
Accordingly, we have found that a need exists for an improved laser-ultrasound device which can detect geometrical, physical, and microstructural properties of an object even more accurately, and which can detect and measure shear resonances in an object.